Television (TV) content distribution is quickly migrating from analog formats to compressed digital formats. Currently, distribution of digital video content for TV display is dominated by use of the MPEG-2 video compression standard (ISO/IEC 13818-2). MPEG-2 and its predecessor MPEG-1 define the standards to compress video content using a combination of various techniques. An MPEG-encoded stream may have three types of pictures, Intra-coded (I), Predicted (P) and Bi-directionally predicted (B). I-pictures are not compressed using any temporal predictions and can be decoded without the need of any other picture. The P-pictures perform temporal predictions from a picture that comes before it in the display order. Thus, decode of a P-pictures requires one picture (from the past) to be available with the decoder for performing temporal predictions. This prediction picture may be either an I-picture or another P-picture. The B-pictures are bi-directionally predicted and, hence, use two pictures for prediction, one from the past and another from the future (in display order).
During normal decode of MPEG streams, video decoders store the last two decompressed I/P pictures in memory. The last I/P picture is used for predicting an incoming P-picture and the last two I/P pictures are used for predicting an incoming B-picture.
However, additional functions allow the user to control the presentation of the video data. These functions include pause, freeze, slow motion, and high speed. The pause function stops the video during video playback. The video does not move forward and the last displayed picture is continuously redisplayed until the user releases the pause. When the user releases the pause, playback of the video resumes from the point where it was paused. The video freeze freezes a picture from a streaming broadcast (in contrast to video from a storage device). Since the video is streaming, when the user releases the freeze, play is resumed from the point where the freeze is released. The video to be displayed between the freeze and the freeze release is lost.
During the slow motion function, the video is displayed at a slower rate, controllable by the user. This can be implemented by repeating pictures. However, where the video comprises interlaced pictures, the display device scans alternating lines during each vertical synchronization pulse. The alternating lines during a first vertical synchronization pulse correspond to either a top field or a bottom field. The alternating lines during a second vertical synchronization pulse correspond to the other of the top field and bottom field. Displaying the top field and bottom fields in alternate order over several vertical synchronization pulses results in a jittery displayed picture.
During the high speed function, the video is displayed at a faster speed. However, because the display device scans the display pictures at the same speed, pictures are skipped to achieve the higher speed. Additionally, because I and P pictures are needed for decoding other pictures, the I and P pictures are decoded. A common scheme for implementing the high speed function involves skipping only B-pictures. However, in cases where B-pictures are back to back, the pictures are not dropped uniformly.
Further limitations and disadvantages of conventional and traditional systems will become apparent to one of skill in the art through comparison of such systems with the invention as set forth in the remainder of the present application with reference to the drawings.